1. Field of the Invention
The present invention relates to image-forming devices, and more particularly to an image-forming device capable of suppressing internal temperature rise in the event when sheet-feed errors occur.
2. Description of the Related Art
Conventionally, image-forming devices operated under an electrophotographic technology include a sheet cassette for accommodating sheets of paper, a sheet-feed roller, a pair of registration rollers and the like. When an image forming operation is performed, the sheet-feed roller is driven to feed the uppermost sheet stacked on the sheet-feed cassette into a sheet conveyance path. The sheet of paper thus fed is subject to registration by the pair of registration rollers disposed behind an image-forming section. That is, the leading edge of the sheet of paper is brought into alignment with the nip portion between the pair of registration rollers. The sheet of paper once subject to registration is further conveyed toward the image-forming section by the registration rollers and the sheet-feed roller. The image-forming section forms an image on the sheet of paper.
Despite the sheet-feed roller being driven properly, the sheet may accidentally be caught by the sheet-feed cassette, disabling the sheet to normally feed. To solve such a problem, Japanese Patent Application Publication No. 6-199469 proposes disposing a sheet-feed sensor in a prescribed position between the sheet-feed roller and the registration rollers for sensing a leading edge of the sheet. When a measured conveyance time T1 is longer than a preset sheet conveyance time T, then it is determined that the sheet-feed error has occurred. When it is the case, an image-forming section halts an image-forming operation and a control section advises the operator to clean the sheet-feed roller.
To perform an image fixing operation on the sheet of paper, the temperature of a heat roller in a thermal fixing section has to be increased up to a predetermined target temperature to thermally fix a toner image on the sheet of paper. Passing the sheet of paper through the thermal fixing section dissipates the heat in the heat roller, causing the temperature of the heat roller to lower. Typically, a temperature sensor is provided in the heat roller. If the temperature sensor indicates that the temperature of the heat roller has increased and exceeded the target temperature, the temperature of the heat roller is lowered to the target temperature.
The temperature of the heat roller starts increasing immediately after the sheet-feed error has occurred because the heat in the heat roller is not dissipated by the sheet of paper passing through the thermal fixing section, resulting in an increase in the internal temperature of the image-forming device. At this time, the temperature sensor senses the temperature rise of the heat roller. However, the temperature sensors are generally incapable of providing a real time temperature output but a brief period of time is needed to reflect the current temperature of the heat roller in the output of the sensor. As such, a problem arises such that the temperature of the heat roller temporarily increases immediately after the sheet-feed error has occurred and accordingly the internal temperature of the image-forming device increases. Another problem is that the increase in the internal temperature of the image-forming device excessively warms up and curls the subsequently fed sheet of paper.